An active-matrix substrate for use in a liquid crystal display device and other devices includes switching elements such as thin-film transistors (which will be simply referred to herein as “TFTs”), each of which is provided for an associated one of pixels. An active-matrix substrate including TFTs as switching elements is called a “TFT substrate”.
As for TFTs, a TFT which uses an amorphous silicon film as its active layer (and will be referred to herein as an “amorphous silicon TFT”) and a TFT which uses a polysilicon film as its active layer (and will be referred to herein as a “polysilicon TFT”) have been used extensively.
Recently, people have proposed that an oxide semiconductor be used as a material for the active layer of a TFT instead of amorphous silicon or polysilicon. Such a TFT will be referred to herein as an “oxide semiconductor TFT”. Since an oxide semiconductor has higher mobility than amorphous silicon, the oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. Also, such an oxide semiconductor film can be formed by a simpler process than a polysilicon film.
Patent Document No. 1 discloses a method for fabricating a TFT substrate including oxide semiconductor TFTs. According to the method disclosed in Patent Document No. 1, a TFT substrate can be fabricated in a reduced number of manufacturing process steps by forming a pixel electrode with the resistance of the oxide semiconductor layer locally lowered.